The Pioneer Spacecraft are reported to be losing velocity leaving some scientists wondering if there is some strange new force at work in the universe that would be pulling the spacecraft back towards the sun.

The Pioneer Spacecraft are much like a weather balloon. Just as a weather balloon has conserved momentum, which you can see by the way that weather balloon rockets upwards in the atmosphere when released, and just as a weather balloon uses its conserved momentum to rise up in a gravitational field, so, too, are the Pioneer spacecraft using conserved momentum to climb upwards out of the sun's gravitational field.

The big question here is 'how much momentum do the spacecraft conserve'. As we know a weather balloon only has enough conserved momentum to rise to a certain height, and as it reaches its potential height in the gravitational field it begins to decellerate, and finally slows right down to a velocity of zero. That is all the conserved momentum the weather balloon carried, and that was as far as that balloon got. How much conserved momentum do the spacecraft carry, and how far will they get.

One thing I am convinced of is that the spacecraft will continue to decellerate, and should they not possess the momentum required to escape the sun's gravitational field, it is possible those spacecraft will become like the weather balloon, and their velocity will drop to zero, and those two spacecraft will park and conserve their momentum by sitting on one spot.